CA2413811C - High modulus, multilayer film - Google Patents

Abstract

A multilayer film (10), comprising, in the following order: (a) a first layer (12) consisting essentially of amorphous polyamide; (b) a second layer (14) adhered to a surface (16) of the first layer (12); and (c) a third layer (18) adhered to a surface (20) of the second layer (14), the third layer (18) comprising at least one member selected from ethylene/vinyl alcohol copolymer, polyamide MXD6, polyamide MXD6/MXDI, polyvinylidene chloride, and polyacrylonitrile.

Description

HIGH MODULUS, MULTILAYER FILM
Background of the Invention The present invention relates to packaging materials of a type employing flexible, polymeric fihns. More specifically, the invention pertains to multilayer packaging fihns used in applications requiring a high degree of dimensional stability, i.e., both low shrinkage and low stretch, at elevated temperatures, and also a relatively low oxygen transmission rate.
Packaging applications requiring dimensionally stable films at high temperatures, e.g., up to about 120 C and sometimes as high as about 130 C, include vertical form-fill-seal (VFFS) packaging for "hot fil' l" products, such as soups, sauces, jellies, beverages, and other liquified foods, which are normally maintained at about 80 C to about 100 C during packaging. As is well known, in VFFS packaging, a flowable product is introduced through a central, vertical fill tube and into a formed tubular fihn having been heat-sealed transversely at its lower end and longitudinally.
After being filled, the package, in the form of a pouch, is completed by transversely heat-sealing the upper end of the tubular segment, and severing the pouch from the tubular film above it, usually by applying sufficient heat to melt through the tube above the newly formed upper heat-seal. If the film from which the package is made does not have sufficient dimensional stability, the package becomes distorted both from the heated product and from the heat-sealing. Not only does package distortion ruin the aesthetic qualities of the package, e.g., by distorting any printed information or other labeling on the package, but it can cause the package to become mis-aligned in the packaging machinery, often resulting in ruined packages and costly downtime in production as misaligned packages become wedged between pieces of the machinery or when heat-sealing/severing equipment inadvertently contacts and melts through the walls of the package instead of sealing and severing at the periphery of the package as intended.
Similar considerations apply in VFFS and horizontal form/fill/seal (HFFS) packaging of flowable particulate products, e.g., shredded cheese, frozen chicken wings and nuggets, etc. Although such products are generally not filled while in a heated state, transverse and longitudinal heat-sealing and heat-severing alone are sufficient to cause package distortion, thereby making a film which is dimensionally stable at elevated temperatures highly desirable for such packaging applications.
Another packaging application for which high-temperature dimensional film stability would be desirable is when filmms are used as lidding materials for:
flexible packages such as thermoformed pockets for, e.g., hot dogs, lunch meats, etc.;
semi-rigid vacuum and/or gas-flushed packages for meat and poultry contained in a foam or other semi-rigid type tray; and rigid packages, e.g., for yogurt, custard and other dairy products contained in a rigid cup-like container. When lidding films are applied to such packages, heat is generally used to seal the film to the thermoformed container, tray, or `10 cup in which the product is contained. Without sufficient dimensional stability, the lidding fihns can either stretch or shrink during the lidding process, resulting in incompletely sealed packages and distorted printed images on the films.
A further process necessitating dimensional stability at elevated temperatures is printing. Maintenance of color-to-color registration on the printing press is important, as is overall consistency of the "repeat length" of each printed image. Drying tunnel temperatures commonly reach temperatures of 200 degrees F (93 degrees C). It is therefore preferred that the film have sufficient resistance to stretching, necking and other types of deformation at these temperatures so that registration is not lost, and that the repeat length of the images are consistently maintained on downstream packaging equipment, where it may again face elevated temperatures as noted above.
Films that are dimensionally stable at high temperatures would generally tend to be relatively stiff at room temperatures. This attribute is highly desirable when the fihn is made into a stand-up pouch for, e.g., soups, sauces, beverages, and particulates, when it is thermoformed into a pocket and lidded, and when it is used as a lidding film. Thus, not only would a film having high-temperature dimensional stability be able to withstand the rigors of the packaging process without distortion, but the resultant package would be stiff, which is advantageous in certain packaging applications such as those listed immediately above.
Another requirement of fihns used in many of the aforementioned packaging applications is a low transmission rate of oxygen in order to preserve and extend the shelf life of packaged food products. For many food products, the oxygen transmission rate (OTR) must be on the order of 40 cc/mZ per 24 hours at 1 atmosphere or less.
In order to achieve the above properties, inany conventional packaging films used for such applications have been laminates, i.e., two or more fihn components that are adhesively bonded together, e.g., biaxially-oriented and heat-set polypropylene, polyester, or polyamide films that are adhesively laminated to a heat-sealable film where one of the laminated film components contains a low OTR material such as polyvinylidene chloride. However, adhesive lamination is expensive, due to the relatively high cost of the adhesives and the extra production steps required to produce the laminate, and the reliability of such adhesives is sometimes suspect, e.g., solvents from printing inks can reduce the bond-strength of the adhesives, leading to delamination. Further, certain types of adhesives contain migratable components that can migrate through films and contact the packaged food products.
Instead of using a laminate, it would be preferred to use a film that is fully coextruded, i.e., formed by extruding two or more polymeric materials through a single die with two or more orifices arranged so that the extrudates merge and weld together into a multilayer structure.
A proposed coextruded film having high-temperature dimensional stability and a low OTR includes a core layer of ethylene/vinyl alcohol copolymer (EVOH) bonded on both surfaces to layers comprising amorphous nylon, which may or may not be blended with a crystalline polyamide such as polyamide 12, 612, 6/66, etc. While atnorphous polyamide is advantageous in that it provides relatively high modulus at high temperatures, thereby imparting high-temperature dimensional stability to a film in which it is incorporated, amorphous polyamide exhibits poor adhesion to EVOH, resulting in a film that will too easily delaminate. Blending crystalline polyamide with amorphous polyamide improves the bond strength to EVOH, but at the cost of greatly decreasing the modulus of the amorphous polyamide layers, and therefore of the entire film, at high temperatures.
Accordingly, there is a need in the art for a fully coextruded, multilayer film having a combination of high-temperature dimensional stability, low OTR, and sufficient inter-laminar bond-strength to be useful for commercial packaging applications.

Summary of the Invention That need is met by the present invention which provides a multilayer film, comprising, in the following order:

a. a first layer consisting essentially of amorphous polyamide;

b. a second layer adhered to a surface of the first layer; and c. a third layer adhered to a surface of the second layer, the third layer comprising at least one member selected from ethylene/vinyl alcohol copolymer, polyamide MXD6, polyamide MXD6/MXDI, polyvinylidene chloride, and polyacrylonitrile. Such materials provide the film with a low oxygen transmission rate, i.e., less than or equal to 30 cc of oxygen per square meter of film per 24 hour period at 1 atmosphere and at a temperature of 73 F (at 0%
relative humidity).

In a more specific aspect, the invention provides a coextruded, multilayer film, comprising, in the following order: (a) a first layer consisting essentially of amorphous polyamide; (b) a second layer adhered to a surface of said first layer, said second layer comprising at least one member selected from the group consisting of a polyamide homopolymer or copolymer having greater than 50 mole percent polyamide 6, a polyamide homopolymer or copolymer having greater than 50 mole percent polyamide 66, an acid-modified polyolefin, an anhydride-modified polyolefin, an ethylene acrylic acid copolymer, an ethylene/methacrylic acid copolymer, and an ethylene copolymer or terpolymer having between 5 and 30 mole % ester mer units; and (c) a third layer adhered to an opposing surface of said second 4a layer, said third layer comprising at least one member selected from the group consisting of an ethylene/vinyl alcohol copolymer, polyamide MXD6, polyamide MXD6/MXDI, a polyvinylidene chloride, and a polyacrylonitrile, wherein:

said second layer is adhered to each of said first and third layers at a bond strength of at least 0.5 lbf/inch, and said film has a storage modulus of greater than 30,000 pounds/in2 at 120 C.

Advantageously, the second layer is adhered to each of the first and third layers at a bond strength of at least 0.5 lbf/inch. In addition, the film has a storage modulus of greater than 30,000 pounds/in2 at 120 C for excellent dimensional stability at elevated temperatures.

Brief Description of the Drawings FIG. 1 is a cross-sectional view of a multilayer film in accordance with the present invention;

FIG. 2 is a cross-sectional view of another multilayer film in accordance with the invention; and FIG. 3 is a graph showing storage modulus vs.
temperature for two films in accordance with the invention.
Detailed Description of the Invention FIG. 1 shows a multilayer film 10 in accordance with the present invention having, in order, a first layer 12 consisting essentially of amorphous polyamide, a second layer 14 adhered to a surface 16 of the first layer 12, and a third layer 18 adhered to a surface 20 of the second layer 14. As used herein, the term "multilayer film" refers to a thermoplastic material, generally in sheet or web form, having one or more layers formed from polymeric or other materials which are bonded together by any conventional or suitable method, including one or more of the following methods:
coextrusion, extrusion coating, lamination, vapor deposition coating, solvent coating, emulsion coating, or suspension coating, but preferably by coextrusion.

Third layer 18 functions as an oxygen barrier layer, and preferably has an oxygen 5 transmission rate (OTR) of less than or equal to 30 cc of oxygen per square meter of film per 24 hour period at 1 atmosphere and at a temperature of 73 F (at 0%
relative humidity). In this manner, the entire film 10 will have such OTR. More preferably, layer 18, and therefore film 10, has an OTR of less than 25 cc/mZ per 24 hours at 1 atm., and more preferably less than 20, more preferably still less thanl5, even more preferably less than 10, such as less than 8, less than 7, or less than 6 cc/m2 per 24 hours at 1 atm.
(73 F and 0% RH). Examples of suitable materials for third layer 18 include at least one member selected from ethylene/vinyl alcohol copolymer (EVOH), polyamide MXD6, polyamide MXD6/MXDI, polyvinylidene chloride (PVDC), and polyacrylonitrile (PAN), including copolymers of PVDC and PAN. EVOH is preferred.

By virtue of first layer 12 consisting essentially of amorphous polyamide, film 10 has a storage modulus of greater than 30,000 pounds/in2 at 120 C (storage modulus being determined in accordance with ASTM 5026-95a). Because of this relatively high storage modulus at temperatures up to 120 C, film 10 has excellent high-temperature dimensional stability. More preferably, the multilayer film 10 has a storage modulus of greater than 40,000 pounds/inZ at 120 C.

Small amounts of additives such as slip or antiblock agents, pigments, processing aids and the like can be included in first layer 12, as long as they do not materially affect the basic characteristics of film 10, e.g., by causing the storage modulus of the film to fall below 30,000 pounds/in2 at 120 C or by adversely affecting the inter-laminar bond-strength of the film. Specifically, crystalline or semi-crystalline polyamide is not present in first layer 12, as this detrimentally affects the modulus of amorphous polyamide, as demonstrated in the Examples below (see Example 3).
As used herein, the term "amorphous polyamide" refers to those polyamides which are lacking in crystallinity as shown by the lack of an endotherm crystalline melting peak in a Differential Scanning Calorimeter (DSC) test (ASTM D-3417).
Examples of such polyamides include those amorphous polymers prepared from the following diamines: hexamethylenediamine, 2- methylpentamethylenediamine, 2,2,4-trimethylhexamethylenediarnine, 2,4,4- trimethylhexamethylenediamine, bis(4-aminocyclohexyl)methane, 2,2-bis(4- aminocyclohexyl)isopropylidine, 1,4-diaminocyclohexane, 1,3-diaminocyclohexane, meta-xylylenediamine, 1,5-diaminopentane, 1,4-diaminobutane, 1,3-diaminopropane, 2-ethyldiaminobutane, 1,4-diaminomethylcyclohexane, p-xylylenediamine, m-phenylenediamine, p-phenylenediamine, and alkyl substituted m phenylenediamine and p-phenylenediamine.
Exainples of amorphous polyamides that can be used also include those amorphous polymers prepared from the following dicarboxylic acids: isophthalic acid, terephthalic acid, alkyl substituted iso- and terephthalic acid, adipic acid, sebacic acid, butane dicarboxylic acid, and the like. The diamines and dicarboxylic acids mentioned above can be combined as desired, provided the resulting polyamide is amorphous. For example, an aliphatic diamine can generally be combined with an aromatic dicarboxylic acid, or an aromatic diami.ne can generally be combined with an aliphatic dicarboxylic acid to give suitable amorphous polyamides. Preferred amorphous polyamides are those in which either the diamine or the dicarboxylic acid moiety is aromatic, and the other moiety is aliphatic. The aliphatic groups of these polyamides preferably contain between 4 and 12 carbon atoms in a chain or an aliphatic cyclic ring system having up to 15 carbon atoms. The aromatic groups of the polyamides preferably have mono or bi-cyclic aromatic rings which may contain aliphatic substituents of up to about 6 carbon atoms.
The amorphous polyamide in first layer 12 preferably has a glass transition temperature of at least about 120 C, and may comprise, e.g., at least one member selected from polyamide 61/6T, polyarnide 66/69/61, and polyamide 66/610/MXD6.
Examples of commercially available amorphous polyamide 61/6T polymers include -GrivoryTM G21 nylon from EMS, SelarTM PA 3426 from DuPont, and NovatecTM X21 from Mitsubishi Chemical.
Another aspect of the present invention is that the second layer 14 is adhered to each of the first and third layers 12 and 18, respectively, at a bond strength of at least 0.51bf/inch (ASTM F 904). More preferably, the bond strength between second layer 14 and first layer 12, and between second layer 14 and third layer 18, is at least about 0.71bWinch.

One of the primary functions served by second layer 14 is as a bonding layer, i.e., to bond first layer 12 with third layer 18. When third (barrier) layer 18 comprises EVOH, for example, the inventors have determined that the bond strength between EVOH and amorphous polyamide is insufficient for commercial packaging applications (see Exalnple 3). Selection of the material used in the second layer 14 to maintain an inter-laminar bond strength of at least 0.5 lbWinch will depend upon the material selected for third layer 18. That is, the material_selected for second layer 14 must be capable of bonding to both amorphous polyamide (first layer 12) and the barrier material present in third layer 18 at respective bond strengths of at least 0.51bf/inch.
Accordingly, when the third layer 18 comprises at least one member selected from ethylene/vinyl alcohol copolymer, polyamide MXD6, and polyamide MXD6/MXDI, the second layer 14 preferably comprises at least one member selected from polyamide homopolymer or copolymer having greater than 50 mole percent polyamide 6, polyamide homopolymer or copolymer having greater than 50 mole percent polyamide 66, acid-modified polyolefins, and anhydride-modified polyolefins.
Preferred polyamides in accordance with the foregoing include polyamide 6, polyamide 66, polyamide 6/66, polyamide 66/6, polyamide 6/69, polyamide 6/12, polyamide 66/6 10, and polyamide 66/69. Preferred examples of acid-modified polyolefins include ethylene/acrylic acid copolymer and ethylene/methacrylic acid copolymer.
Preferred anhydride-modified polyolefins include anhydride- grafted ethylene/ I -butene copolymer, anhydride-grafted ethylene/1-hexene copolymer, anhydride-grafted ethylene/1-octene copolymer, anhydride-grafted ethylene/vinyl acetate copolymer, and anhydride-grafted polypropylene.
As used herein, the term "anhydride-rnodified" refers to any form of anhydride functionality, such as the anhydride of maleic acid, fumaric acid, etc., whether grafted onto a polymer, copolymerized with a polymer, or blended with one or more polymers, and is also inclusive of derivatives of such functionalities, such as acids, esters, and metal salts derived therefrom.
When the third layer 18 comprises at least one member selected from polyvinylidene chloride and polyacrylonitrile, the second layer 14 preferably comprises an ethylene copolymer or terpolymer having between 5 and 30 mole % ester mer units.

Such ester mer units may include, for example, methacrylate, n-butyl acrylate, iso-butyl acrylate, ethyl acrylate, vinyl acetate (e.g., ethylene/methacrylate copolymer, etc.). An example of a suitable terpolymer may comprise 80% ethylene, 10% iso-butyl acrylate, and 10% methacrylic acid.
Multilayer film 10 may be made by any suitable and known film-making process, e.g., cast or blown through either an annular or flat die, and is preferably fully coextruded. As used herein, the term "coextruded" refers to the process of extruding two or more materia.ls through a single die with two or more orifices arranged so that the extrudates merge and weld together into a laminar structure before chilling and solidifying, e.g., via water, chilled metal roll, or air quenching. Multilayer film 10 preferably has less than 5% shrink in at least one direction at 120 C. Thus, it is preferred that the filxn is not oriented after coextrusion or, if it is, that it be heat-set to remove substantially all of the shrink capability from the film. This property, i.e., less than 5% shrink at 120 C, along with a storage modulus of greater than 30,000 pounds/in2 at 120 C, results in a film with excellent dimensional stability, both at room temperature and at higher temperatures, e.g., up to about 120 C, as encountered during many food packaging processes as discussed hereinabove.
Multilayer film 10 may have any desired thickness, e.g., ranging from about 0.5 to about 50 mils. More preferably, the thickness ranges from 0.5 - 40 mils, more preferably still from about 0.5 - 30 mils, yet more preferably from about 0.5 -20 mils, even more preferably from about 1 - 15 mils, still more preferably from about mils, and most preferably from 1 - 5 mils, such as from 1.5 - 4 mils.
Referring now to FIG. 2, a nine-layer embodiment of a multilayer fihn in accordance with the present invention will be described. Multilayer film 10 may be used alone, i.e., as a three-layer film. For certain packaging applications, it may be beneficial to include the three-layer film 10 as a sub-structure in a larger multilayer film. In such a configuration, the larger multilayer fihn incorporating film 10 as a sub-structure would possess all of the properties and benefits resulting from the three-layer structure of film 10 as discussed above, but would also possess additional properties and benefits arising from the additional layers.

Mu]tflsyer flm 22 in f+7(T. 2 is an examp'ia of aucb a lerger multilayec fli m, ooni a;, layers 12. 14r attd 16 as interiox layecs. Tb,at is, in multilayrr Slm 2Z, sccowA
layer 14 is adhcrod to surfaoc 16 of first layer 12 (consist+ng essentieU,y of amorl,hous polyysaude), and third (batiiar) layar 18 is adhored to surfaoo lU of aecmn-d layer 14. In film 22, however, all tt,oree lal-ars 12, 14, aad 18 arc iul.ca-ior layers. As n9ed herein, the phrase'Intmrior 1aye~' refers to any ]ayer of a szmltdaycr f ilm haYing botti of its prpacipal soxfaces directl,y aeihexed to anothex layer of the fi1m.
Mults`laycr 11m 22 preferably tutudes a Rbuzth, extesior layer 24 compri,siag a matecial thnt is capable of forrning e, heat-seal. As used he-ein, the phrase "exterior 30 layerzs rotm to aay laycr of a aaaltilayar filnn, bAviag only one of tts princiral awrfaoe+a dicP.aCy aclhered to saoth r layar of thaE fibn Iu a innlttlayer film, thaCC
are two Werior layecs, each of wbich has a principal surface adbered to 0uly onc othcr layer of tbte 1ilm.
Tbz othar principal sual'ace of eaoh of the two sxterior lsyars forms the two priuca,pui outcr suzfaoes oftiie fhn6 i5 As used hrercin, tIw tcnm "heat-soar' rdkrs to the uuinn of two fiLrns by bczngbt.g the fikas iinfo contaot, or at lea9t close pmdazity, willt on4 auother and then apply5ng sufflci:ent beat and pressnre to a predetermined area (or arcas) of the Mns to cau9e the uoni.ac. soafbce3 of the flms ia the predetermmsd area to bocomc molteu ewd intecm~ with onc auuihCr, thereby fotming an esxmtiaPy =eparabi,e bond bctwoca'the 20 two fitets in the predetcraunad arca vt+hvu the heat and pressnre are removed thorcfiom and t.he area is allowed to cooL
Bxamples Qf suitable materials for fourth, cxtcrior layer 24 mclude lonomers TM
(e,g., OURLYN $om DuPont), betmgen.aoue (Zeigez Naft catatyr,ed) athyIva/4lta-oLca copotym,ar snoh as Bum low deiusiry polyetlaylene, homogaaeous (raatslloccnc or 25 siug,lo-sito catalyCOd) cthylenrJalpIki.ole8n eopolymez, ethyleD.e/vmyl.
goetate copolynlec, etklenelmefilayl aorylato oopolYmracr o"Me/pTopylene cupolrv,er, pnlybutylene hnruopotymer or copotymw, aad ot,her siuular materiats that are capable nf orruaq,g a heai-seal Muli.&yer fl1m. 72 pareibrably fnzther comprbes a fifth, aaterior ltiyar 2G
9U aosnpxi,sing a material b,ta,ving a surface energy of at least 36 dypeshm'.
Such a emr!'aco e=BY allowe 9n mmgc to be printed on outer suriacc 29 of fsfth layat 2fi_ Suitable ..._...~ _, __ materials for fifth layer 26 include at least one material selected from polyamides, polyesters, polycarbonates, polyurethanes, and polyacrylates. Polyolefin surfaces having been modified by corona, plasma, flame treatment or acid etching such that the surface energy exceeds 36 dynes/cmZ are also suitable for printing. Fifth layer 26 also preferably 5 provides heat- and abuse-resistance to the film. A semi-crystalline polyarnide such as polyamide 6 is preferred.
Multilayer film 22 may fixrther comprise a sixth, interior layer 30 positioned generally between the third and fourth layers 18, 24, e.g., adjacent fourth layer 24 as shown. It is preferred to include sixth layer 30 to balance any tendency of fifth layer 26 10 to cause the film 22 to curl, which may occur if layer 26 contracts to a greater or lesser degree than the other layers of the film upon cooling. For example, when fifth layer 26 is a crystalli.ne or semi-crystalline material, such as a semi-crystalline polyamide, it contracts to a greater degree upon cooJ.ing than the other film layers and, being an exterior layer, exerts a curling force on the film such that the film tends to curl towards the fifth layer. This tendency can be offset by including in sixth layer 30 a material that balances the crystallinity of fifth layer 26, i.e., a crystalline or semi-crystalline material.
Thus, sixth layer 30 preferably comprises at least one material selected from high density polyethylene, low density polyethylene, linear low density polyethylene, polypropylene, polyamide, and polyesters. High density polyethylene is preferred because it has sufficient crystallinity to balance that of a semi-crystalline polymer in layer 26, such as polyarnide 6, and also adheres well to materials from which the fourth (heat-seal) layer 24 is preferably constructed.

Multilayer film 22 may further comprise a seventh layer 32 adhered to a surface 34 of third layer 18. The seventh layer 32 may be formed from the same material(s) as second layer 14, and thus may comprise at least one member selected from polyamide homopolymer or copolymer having greater than 50 mole percent polyamide 6, polyamide homopolymer or copolymer having greater than 50 mole percent polyamide 66, acid-modified polyolefins, anhydride-modified polyolefins, ethylene/acrylic acid copolymer, ethylene/methacrylic acid copolymer, and ethylene copolymer or terpolymer having between 5 and 30 mole % ester mer units. Preferred materials include at least oue mamber selected fiom polyaminde 6, po4wnidc 66, polyamidc 6/66, and polyamide 66/6.
Potymeria adhcsive kyess may be inclacletl io, aiaWayaor film 22 w&=cvcr aecessary or desircd. T1u6, a first pulyyumlc adbuive layer 36 uiay'be inolodcd between 6 fnrt layer 12 and filth layct 26 and a sccuut] polymerlc adbG4i.ve layer-38 ma.y be lpehuied betvueen si-tth layer 30 and scvcntb, laycr 32. T.a,yczs 36 and 38 may cnmpriae any saitable potymeric adhesive, saeh as an nlefin polytxxcr ur cupolymet having an trv}ylride fiaw'tlop8lW Etntted theraan and/or cepolyavxizsd ttiesvwith and/vr blen,ded therowOz. ExasxOes of suitablg po]ytnerio adhEsives asalade the fo7Lowmg an'hydrdo-1 U grafted pob-aieCs: othy1caac/1=buteue aopo],y=, ctbyLeaell-hexene cokoly:aer, athylenedlrocteua eopol,y-uie+r, athykaclvinyl aactectv 4vpolymec, po'lypropylene, and low dewity polyetbylene_ =
T-Le aiv=tion w01 xLow be fuefter desectibed tn the fnIlowiug emwp1;s, 15 Rx~-~vcptMt~
The materisils nsM In the eaaaaples are idontfied be]ow;
TM
1. P-AW6-1: CAPRON 13 100 WP; a polyamide 6,resia baving a deoaiiy of 1.135 glcc (ASTM 1305) and a melting point of 220 C; purohoaod from kioneywCu of HQpewelL, VA.
TM
20 Z. ak,A.: GRIVORY G21; aa euufiphous pobwUde with a gisss trAnaitioa ternperature of 125 C apd a dcasity of 1.18 glco (ASTM 1505); obtainrA frpm BMS of Sum.c.er, 51C
TM
3. PA6-2: ULTRAlYIID KR-4407: g polycomde 6 resia baving e, dmsity of 1.14 g/vc (ASTM 1505) a,nd anadft point of 220 C, coutaiaing less than 1%, combin+ed lal,C
25 aad si7icon oi'(. ,- purcbasod ftm$AsF of Momnt Olive, NJ.

4. PA666; TTT,TRAMID C3S; a polyaIItide 6166 copolymer ltaving a debsity of 1.14 g/cc; purch6Sed froin EiASF ofItQaunt Oliva, NJ.
TM
5, MB1: GJZII.ON XE 3361; a polyamide 6 masterbateh contnaming 5% taL-un (magnesiarn sZicaie), 5% r.alcium caibouateõ atld 5% p,n'-e.ftlee bas ataaramidc with a demtsfty of 1.140 p, f oc (A-4'1'Ad 1505) and a malting poim of 2?_0 G; pwrcbased fiom EMS of SumLer, SC
TM
6. M$2: (:fr.ARIANT 1080864S; a polytsmide 6 Tnasterb.atch oontabqiug 20%
diatomaceou.a earth and 101IMP ern.camcidc with a dwsity of 1.20 g/cc (ASTlvi 1505) g and a=ltiug point of 220 C; purobascd fnam C1ari.anC of Minnesnolis, MN.
TM
7. MB3: .H'MPACLrl 10853: a tinear low deasity polyothylCCUr ba8ed msstetbatch contaiamg 19.4% diatomavcou5 earth with a melt index of 1.5 glain (ASTM 1238) azi,d a ciensfty of 1.00 g/oc (ASTM 1505); obtained fi~om .Ampacat of Y1e Ridder, LA.
TM
S. MB4: FSV 255E; a.t DPE-based maslerbatoh cuntabJqg 25 /n diatomaccous siliCa and 5% erucaznide; obtained fmm Sehalmer.
TM
9. FVOH: SOARANOL ET; tm e?bylenPJvia.yl alcobol oopolym,cx haveng 38"/n ethyleae, a mwlt iadex of3.2 (AST.M 123 8) anrt a density of 1, 17 g/ov (A,STM
1305); obtainerl fr+orn Nippon Goahei, supplied by Soarns of Ax}mgton Hcight,y, IL.

10.1ff.,1: TYM4R 122i1l3, an eahydride-grnltcd polyvlefnri resm havtng a uiett bod.ex of 2.1 (ASTM 1238) and a density of 0.921 g,loo (Ab I'M 1505);
pntcltaved frnm RDhm aud Huas of Ph0adalphis, PA.
TM
11.17E-2: BYML CX A 398660; au anhydtido-grafted polyolefia in ethyleo.e/viqyl acetsr.e }igving a znoU IDdex of 2.4 (ASTM 1238) and a, c3cnsitV of 0_943 glen (ASTM
1505); alytaioned fromDuPout ofW7mftroa, Dil.
TM
12. .HDPE: FC]R.i'1,FI,EX 760-500=119; a high deu.ait,y polyethylene wft a ma7t iadr:~, of 6-2 g/moin (ASTM 1238) and a don9ity of 0961(ASTM 1505); obtaiaed fnrm Soli-a.y poiyo=s of I3ouston. 'rX.
TM

13 _ C,LDPE: BXCE$D 36l C33; a, homogeneous ethylene hoxenc-1 uc,poXyMex with a melt mde,x of4.5 ghnm. (ASTM 1236) and a density of 0.917 g/oc (AS4 lv! 1505) purclased from F-man Nlobg ofHouskan, TX.
TM

14. mru: EscoRBivB Lmoo.48; a low dcnsity i,olyeth.ylcaa with a melt in,dex of 7.5 g/mia (ASTM 1238) and a density of 0.917 g/oc (ASTM 1505); oiytained itnm Exuon 114obil afHowshon, TX.

. _,._ _.~..-_~,~.._......._.y..._u..w...~. .__....._ . _ _ _ Example 1 With reference to FIG. 2, a multilayer film in accordance with the present invention had the following nine-layer structure (total film thickness ranging from 3.3-3.5 mils):

Layer 24: 88% LLDPE + 10% LDPE + 2% MB3 (20% of total thickness of layers 1-9), Layer 30: HDPE (19% of total thickness of layers 1-9).
Layer 38: TIE-1 (8% of total thickness of layers 1-9).
Layer 32: PA6-1 (6.5% of total thickness of layers 1-9).

Layer 18: EVOH (6% of total thickness of layers 1-9).
Layer 14: PA6-1 (6.5% of total thickness of layers 1-9).
Layer 12: aPA (10% of total thickness of layers 1-9).
Layer 36: TIE-2 (11 % of total thickness of layers 1-9).

Layer 26: 96% PA6-2 + 2% MB1 + 2% MB2 (13% of total thickness of layers 1-9).

The film was fully coextruded and produced by a cast coextrusion process using an annular die, followed by a water quench upon exiting the die.

Example 2 Again with reference to FIG. 2, a multilayer film in accordance with the present invention was made as described in Example 1 and had the following nine-layer structure (total film thickness ranging from 3.6-3.7 mils):

Layer 24: 96% LLDPE + 4% MB4 (15% of total thickness of layers 1-9).
Layer 30: HDPE (18% of total thickness of layers 1-9).

Layer 38: TIE-1 (6% of total thickness of layers 1-9).
Layer 32: PA6-1 (6.5% of total thickness of layers 1-9).
Layer 18: EVOH (6% of total thickness of layers 1-9).

Layer 14: PA6-1 (6.5% of total thickness of layers 1-9).

Layer 12: aPA (18% of total thickness of layers 1-9).
Layer 36: TIE-2 (14% of total thickness of layers 1-9).

Layer 26: 96% PA6-2 + 2% MB1 + 2% MB2 (10% of total thickness of layers 1-9).

The fihns of Examples 1 and 2 where subjected to the following tests:

1) Normalized Impact Total Energy (NITE); tested in accordance with ASTM D
3763;
expressed in ft-lb per mil of film thickness.

2) Storage Modulus; tested in both the machine direction (MD) and transverse direction (TD) in accordance with ASTM D 5026-90; expressed in psi (pounds/in).

3) Curl Rating; a subjective observation of each fi1m.'s tendency to curl or lay flat when placed on a flat surface at room temperature; a scale of 1 to 5 was used with "1"
indicating excellent flatness and "5" indicating highly problematic curling.

4) Shrinkage; tested in accordance with ASTM D 2732 at 150 C.

5) nxygen Transmission Rate; tested in accordance with ASTM D 3985; expressed as cc of oxygen per square meter of film per 24 hour period at 1 atmosphere and at a temperature of 73 F (at 0% relative humidity).

The results are summarized in Table 1.

Test Example 1 Example 2 NITE (ft-lb/mil) @ 73 F 0.155 0.194 NITE (ft-lb/mil) @ 41 F 0.086 0.103 NITE (ft-lb/mil) @ 32 F 0.082 0.084 Storage Modulus at 80 C MD/TD 65/71 82/84 (psiX1000) Storage Modulus at 100 C 57/57 78/76 MD/TD (psiX1000) Storage Modulus at 120 C 48/47 65/62 MD/TD (psiX1000) Storage Modulus at 140 C 12/15 11/14 MD/TD (psi.Xl000) Storage Modulus at 150 C 10/10 8/8 MD/TD (psiX1000) Curl Rating (1 [excellent] - 5[poor]) I I
Shrinkage LD/TD (%) 0/0 0/0 Oxygen Transmission Rate 5.41 4.58 As indicated in Table 1, the fihns of Examples I and 2 maintain high modulus, i.e., greater than 30,000 psi, at temperatures exceeding 120 C, i.e., up to about 130 C.
This is illustrated graphically in FIG. 3 (machine direction only). In addition, both filrns 5 exhibited 0% shrink at 150 C, and therefore have 0% shrink at lower temperatures, e.g., at 130 C, 120 C, etc. These properties, i.e., high modulus and no shrink at elevated temperatures, result in excellent dimensional stability at high temperatures such as are experienced during various packaging operations as discussed hereinabove.
While fihns having such a high modulus might suffer the drawback of being overly 10 brittle, the iinpact test results (NITE) indicate that the films are sufficiently ductile throughout the tested temperature range for commercial packaging applications.
The data in Table 1 also indicates that the fihns have excellent flatness, i.e., a very low tendency to curl. This greatly facilitates the handling and processing of the films during packaging, particularly as the films are being moved and manipulated in the 15 packaging machinery. Finally, the films also exhibit a very low OTR, a highly beneficial property for the packaging of perishable items such as food products.

Example 3 In order to determine the effects of blending amorphous polyamide with crystalline polyamide on both (1) bond strength with EVOH and (2) modulus, various two-layer fihns containing a 1 niil layer of EVOH bonded to a 4 mi.l layer of either amorphous polyamide, polyamide 6/66 copolymer, or blends thereof, were constructed by cast coextrusion. The structure of those fihns may be summarized as follows (where a"/" indicates the interface of the two layers and all percentages are weight percentages):

Sample 1: 100% aPA/lEVOH

Sample 2: 90% aPA + 10% N666/EVO1FI
Sample 3: 75% aPA + 25% N666/EVOH
Sample 4: 50% aPA + 50% N666/EVOH
Sample 5: 100% N666/EVOH

Each of the film samples was tested for interlayer bond-strength (ASTM F 904), Young's modulus at 25 C (ASTM D 882), and storage modulus (ASTM 5026-90) at various temperatures. The bond-strength and Young's modulus data is summarized in Table 2.

SAMPLE BOND ST.RENGTH YOUNG'S MODULUS
(lbf/inch) (psi) 1 0.03 346,000 2 0.08 335,000 3 0.46 343,000 4 Inseparable 220,000 5 (not tested) (not tested) Films having interlayer bond-strengths of less than about 0.5 lb,inch are generalty considered unsuitable for commercial packaging applications. Thus, the fihns of Samples 1, 2, and 3, having 100%, 90%, and 75% amorphous polyamide, respectively, would not be suitable for commercial packaging use, whether such fihns were used alone or as a sub-component of a larger, multilayer film. In other words, Applicants have found that film layers containing more than 50% amorphous polyamide in a blend with polyamide 6/66 or other crystalline polyamide, have an inadequate bond strength with EVOH. Only when the amount of amorphous polyamide decreases to below about 50% does the bond-strength increase sufficiently to be acceptable for commercial use.

However, as also indicated in Table 2, the Young's modulus of the films decreased with increasing amounts of polyamide 6/66 blended with the amorphous polyamide. This phenomenon was shown much more clearly when the film samples underwent storage modulus testing at increasing temperatures ranging from 0 C
to 146 C as indicated in Table 3 below.

STORAGE MODULUS (PSI) Temp 100% aPA 90% aPA + 75% aPA + 50% aPA + 100%
( C) 10% PA666 25% PA666 50 % PA666 (Sample 1) (Sample 2) (Sample 3) PA666 (Sample 4) (Sample 5) 0 415,000 409,000 258,000 332,000 380,000 5 408,000 402,000 250,000 328,000 361,000 10 399,000 394,000 244,000 327,000 336,000 391,000 386,000 239,000 325,000 305,000 383,000 377,000 235,000 317,000 270,000 374,000 368,000 233,000 282,000 240,000 362,000 354,000 236,000 219,000 207,000 342,000 325,000 237,000 123,000 137,000 328,000 298,000 251,000 81,403 103,000 314,000 282,000 233,000 64,956 86,504 300,000 262,000 187,000 53,338 75,179 285,000 242,000 134,000 44,933 65,353 275,000 227,000 96,702 38,197 58,159 269,000 205,000 77,055 33,279 52,725 70 262,000 177,000 65,747 27,499 49,270 76 254,000 150,000 54,798 22,832 47,043 81 242,000 129,000 46,353 18,976 45,383 86 231,000 120,000 38,822 16,264 44,385 91 222,000 114,000 31,137 15,089 43,315 96 223,000 116,000 23,956 14,714 41,942 101 230,000 132,000 16,041 14,680 40,362 105 231,000 150,000 9,533 14,090 38,043 111 233,000 145,000 6,571 12,812 35,440 116 207,000 111,000 5,414 11,286 33,229 121 206,000 55,095 4,666 10,316 31,095 126 200,000 13,945 4,000 9,939 29,138 131 139,000 4,838 3,443 9,752 27,462 136 37,221 3,403 3,071 9,440 25,646 141 4,188 2,736 2,499 8,937 23,904 146 2,338 2,223 0 8,522 22,006 As shown by the results in Table 3, the storage modulus of samples 2-5 decrease dramatically relative to sample 1 as the temperature of the film samples increases. This is particularly noteworthy in a comparison of sample 1(100% amorphous PA/EVOH) with sample 2, which is the same as sample 1 except for the addition of 10% PA

copolymer with the amorphous PA layer. Between about 116 and 121 C, the modulus of sample 2 dropped steeply to 55,095 psi and then to 13,945 psi between 121 and 126 C. In contrast, the modulus of sample 1 remained well above 100,000 psi even as the temperature increased above 130 C. Accordingly, a layer consisting essentially of 100% amorphous polyamide is important in achieving high-temperature stiffness/dimensional stability. Even the addition of as little as 10%
crystalline polyamide has a dramatic detrimental effect on the high-temperature modulus property of the amorphous PA layer.

Since the amorphous PA-containing layer is principally responsible for providing high temperature stiffness to the multilayer fihns of the present invention, which stiffness effect is diminished by the other lower modulus layers of the film, it is preferred not to blend crystalline polyamide with the amorphous polyamide when high temperature dimensional stability is desired of such multilayer film. At the same time, since amorphous polyamide has been determined to exhibit very poor bonding with EVOH, it is highly advantageous to include a suitably strong bonding layer, such as crystalline' polyamide, between the amorphous PA and EVOH layers in accordance with the present invention to obtain iinproved bond strength without sacrificing high-temperature modulus.
While the invention has been described with reference to illustrative examples, those skilled in the art will understand that various modifications may be made to the invention as described without departing from the scope of the clairns which follow.

Claims (17)

CLAIMS:

1. A coextruded, multilayer film, comprising, in the following order:

(a) a first layer consisting essentially of amorphous polyamide;

(b) a second layer adhered to a surface of said first layer, said second layer comprising at least one member selected from the group consisting of a polyamide homopolymer or copolymer having greater than 50 mole percent polyamide 6, a polyamide homopolymer or copolymer having greater than 50 mole percent polyamide 66, an acid-modified polyolefin, an anhydride-modified polyolefin, an ethylene acrylic acid copolymer, an ethylene/methacrylic acid copolymer, and an ethylene copolymer or terpolymer having between 5 and 30 mole % ester mer units; and (c) a third layer adhered to an opposing surface of said second layer, said third layer comprising at least one member selected from the group consisting of an ethylene/vinyl alcohol copolymer, polyamide MXD6, polyamide MXD6/MXDI, a polyvinylidene chloride, and a polyacrylonitrile, wherein:
said second layer is adhered to each of said first and third layers at a bond strength of at least 0.5 lbf/inch, and said film has a storage modulus of greater than 30,000 pounds/in2 at 120°C.

2. The multilayer film of claim 1, wherein said amorphous polyamide in said first layer has a glass transition temperature of at least about 120°C.

3. The multilayer film of claim 1 or 2, wherein said amorphous polyamide in said first layer comprises at least one member selected from the group consisting of polyamide 61/6T, polyamide 66/69/61, and polyamide 66/610/MXD6.

4. The multilayer film of any one of claims 1 to 3, wherein said third layer comprises at least one member selected from the group consisting of an ethylene/vinyl alcohol copolymer, polyamide MXD6, and polyamide MXD6/MXDI.

5. The multilayer film of any one of claims 1 to 3, wherein said third layer comprises at least one member selected from the group consisting of a polyvinylidene chloride and a polyacrylonitrile.

6. The multilayer film of any one of claims 1 to 5, wherein said second layer comprises at least one member selected from the group consisting of polyamide 6, polyamide 66, polyamide 6/66, polyamide 66/6, polyamide 6/69, polyamide 6/12, polyamide 66/610, and polyamide 66/69.

7. The multilayer film of any one of claims 1 to 6, wherein said film has less than 5% shrink in at least one direction at 120°C.

8. The multilayer film of any one of claims 1 to 7, wherein said film has a storage modulus of greater than 40,000 pounds/in2 at 120°C.

9. The multilayer film of any one of claims 1 to 8, further comprising a fourth, exterior layer comprising a material that is capable of forming a heat-seal.

10. The multilayer film of claim 9, further comprising a fifth, exterior layer comprising a material having a surface energy of at least 36 dynes/cm2.

11. The multilayer film of claim 10, wherein said fifth layer comprises at least one material selected from the group consisting of a polyamide, a polyester, a polycarbonate, a polyurethane, and a polyacrylate.

12. The multilayer film of claim 10 or 11, further comprising a sixth layer positioned between said third and fourth layers, said sixth layer comprising at least one material selected from the group consisting of a high density polyethylene, a low density polyethylene, a linear low density polyethylene, a polypropylene, a polyamide, and a polyester.

13. The multilayer film of claim 12, further comprising a seventh layer adhered to a surface of said third layer, said seventh layer comprising at least one member selected from the group consisting of a polyamide homopolymer or copolymer having greater than 50 mole percent polyamide 6, a polyamide homopolymer or copolymer having greater than 50 mole percent polyamide 66, an acid-modified polyolefin, an anhydride-modified polyolefin, an ethylene/acrylic acid copolymer, an ethylene/methacrylic acid copolymer, and an ethylene copolymer or terpolymer having between 5 and 30 mole % ester mer units.

14. The multilayer film of claim 13, wherein said seventh layer comprises at least one member selected from the group consisting of polyamide 6, polyamide 66, polyamide 6/66, and polyamide 66/6.

15. The multilayer film of any one of claims 1 to 14, having a thickness ranging from about 0.5 to about 50 mils.

16. The multilayer film of any one of claims 1 to 15, wherein said film has an oxygen transmission rate of less than or equal to 30 cc of oxygen per square meter of film per 24 hour period at 1 atmosphere and at a temperature of 73°F at 0% relative humidity.

17. The multilayer film of claim 16, wherein said film has an oxygen transmission rate of less than or equal to 15 cc of oxygen per square meter of film per 24 hour period at 1 atmosphere and at a temperature of 73°F at 0%
relative humidity.